Generally, an integrated circuit has to receive various supply voltages. A charge pumping circuit is a voltage generator that is installed in the integrated circuit. According to the practical requirements, the voltage generator provides various voltages. Generally, according to a pumping clock signal, a specified voltage is increased to a higher pumping output voltage in a stagewise fashion by the charge pumping circuit.
FIG. 1A is a schematic circuit diagram illustrating a conventional charge pumping circuit. FIG. 1B is a schematic timing waveform diagram illustrating associated signals processed by the charge pumping circuit of FIG. 1A.
As shown in FIG. 1A, the charge pumping circuit 100 comprises a pumping unit 102 and a feedback detector 104. A pumping clock signal CLKpump is received by a control terminal Cr of the pumping unit 102, and a pumping output signal Vpump is outputted from an output terminal O of the pumping unit 102. Moreover, the output terminal O of the pumping unit 102 is connected with a capacitor C.
Generally, the pumping output signal Vpump is controlled according to the pumping clock signal CLKpump. That is, the voltage level of the pumping output signal Vpump is gradually increased when the pumping clock signal CLKpump is activated; and the voltage level of the pumping output signal Vpump is gradually decreased when the pumping clock signal CLKpump is inactivated. Consequently, the pumping clock signal CLKpump is selectively activated or inactivated by the feedback detector 104 of the charge pumping circuit 100.
The feedback detector 104 comprises a voltage divider 106, a comparator 107, and an AND gate 108. The voltage divider 106 comprises two resistors R1 and R2. The two resistors R1 and R2 are connected between the output terminal O of the pumping unit 102 and a ground terminal GND. A feedback voltage Vfb is outputted from a node a between the two resistors R1 and R2. Moreover, the feedback voltage Vfb is inputted into a negative input terminal of the comparator 107. A reference voltage Vref is inputted into a positive input terminal of the comparator 107. Moreover, a first input terminal of the AND gate 108 is connected with an output terminal of the comparator 107, a reference clock signal CLKref is received by a second input terminal of the AND gate 108, and the pumping clock signal CLKpump is outputted from an output terminal of the AND gate 108.
If the feedback voltage Vfb is lower than the reference voltage Vref, the comparator 107 outputs a high level to the AND gate 108. Consequently, the clock signal CLKpump is activated. On the other hand, if the feedback voltage Vfb is higher than the reference voltage Vref, the comparator 107 outputs a low level to the AND gate 108. Consequently, the clock signal CLKpump is inactivated. Moreover, the relation between the pumping output signal Vpump and the reference voltage Vref may be expressed by the following equation:
  Vref  =      Vfb    =                  (                              R            ⁢                                                  ⁢            2                                              R              ⁢                                                          ⁢              1                        +                          R              ⁢                                                          ⁢              2                                      )            ×      Vpump      
Please refer to FIG. 1B. Since the pumping clock signal CLKpump is selectively activated or inactivated under control of the feedback detector 104, the pumping output signal Vpump outputted from the pumping unit 102 fluctuates relative to an average voltage Vavg in a DC form. Consequently, Vpump=Vavg+Vripple, wherein Vripple is the amplitude of a ripple voltage in an AC form.
For allowing the charge pumping circuit 100 to generate a predetermined value of the average DC voltage Vavg, the reference voltage Vref should be manually adjusted before the IC chip leaves the factory. That is, the pumping output signal Vpump is changed until the average DC voltage Vavg of the pumping output signal Vpump reaches the predetermined value.
Obviously, the way of manually adjusting the pumping output signal Vpump is time-consuming and labor-intensive.